Charge ordering in the layered Co-based perovskiteHoBaCo2O5

From neutron powder diffraction studies performed on the ordered oxygen deficient perovskite ${\mathrm{HoBaCo}}_{2}{\mathrm{O}}_{5},$ we have observed two structural phase transitions at ${T}_{N}\ensuremath{\sim}340$ K and ${T}_{\mathrm{CO}}\ensuremath{\sim}210$ K, corresponding to the occurrence of magnetism and the onset of charge ordering, respectively. The charge ordered phase exhibits chains of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ along the a axis and an alternate stacking of these ions along the b and c axes. The observed magnetic moments, ${\ensuremath{\mu}}_{{\mathrm{Co}}^{3+}}=3.7{\ensuremath{\mu}}_{B}$ and ${\ensuremath{\mu}}_{{\mathrm{Co}}^{2+}}=2.7{\ensuremath{\mu}}_{B},$ confirm this charge ordering and are in good agreement with the expected values for high-spin Co ions in pyramidal environment.